PowerPoint Slideshow about 'Chapter 3' - Samuel

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A load is considered primary or secondary according to the part of the bridge which shall be designed. Wind loads are secondary loads in designing the main girders and primary loads in designing the wind bracings.

The type of trains is different for different countries according to the importance of lines. In Egypt we shall consider one type of three train types (D, H, and L), train type “D” is the heaviest train is used in Egypt.

Train type “D” consists of a two locomotives and two tenders followed on one side only by an unlimited number of wagons

Within the kerb to kerb width of the roadway, the standard vehicles are assumed to travel parallel to the length of bridge, and to occupy any positions, which produce the maximum stress. For the standard vehicle, all the axles of a unit of vehicle are considered as acting simultaneously in a position causing maximum stresses. The vehicle in adjacent lanes is taken as headed in the direction producing maximum stresses. The maximum bending moment and maximum shear force on the plate girders are found by longitudinal location of loading.

For main roadway bridge, the L.L shall be that type of vehicular rolling load and/ or distributed load representing it.

As well as the floor system shall be designed for truck concentrated axle loads the standard type shown together with distributed load of 500 kg/m2 cover the main lane of three meter width lane(p5-1), the second lane of three meter width each shall be covered with one truck moving in the same direction and parallel to the axes of the bridge. The remaining parts of the floor are covered with a uniform load of 300 kg/ m2 (p5-3). Also, side walks shall be covered by the same distributed load. The impact will be considered for the loads on the main lane only.

It shall be designed for uniform load of 500 kg/ m2, without impact(5-2-4).

3.4 Impact loads

Impact is the dynamic effect on the bridge due to the moving loads. If we measure the deflection at a certain point of the bridge for slowly moving train (static L.L), and for rapidly moving train (static L.L + Impact), the increase of deflection in the later case is due to impact.

1.Roughness and unevenness of the track of a railway bridge or of the roadway surface of a roadway bridge. The smoother of the surface the smaller will be the impact. In Railway Bridge the joints of rails increase the impact. It is recommended to use long rails on bridges or to weld the joints.

For bridges in curves, the stresses due to the centrifugal action and the super elevation of the track must be considered in designing the members. A vertical load w moving in a curve of radius R and a speed V.

C is a horizontal force acting at the center of gravity of masses 2.00 above the top of rail (6.4.1) . It produces an increase of the vertical reaction on the outside rail and a decrease of the vertical reaction on the inside rail.

When steel structure is not free to expand or contrast under variation of temperature, the stresses due a variation of  30 C. From local main must be considered. The coefficient of expansion for steel and concrete is 0.00001. If we consider unequal variation of temperature, in some structures which are not affected by equal changes, we allowed only for  15 C.

In two hinged arches and suspension bridges the equal change of temperature has an effect on the internal forces. In continuous bridges the equal change of temperature has no effect because the girders are free to expand, but the unequal change has an effect.

For bridges we consider either the case of unloaded bridge with a wind pressure of 200 kg/m2 or the case of loaded bridge with a wind pressure 100 kg/m2 on exposed surfaces of bridge and train. The effective height of a train in railway bridges is 3.50 m from the rail level, and that for crowds or road vehicles is 3.00 m. The train is considered as having on single vertical plane only.

In railway bridges (6-6) we have to consider the stresses resulting from the application of brakes to the live load while passing on the bridge. The braking force is equal to 1/7 of the maximum Live Load, without impact, supported by one track only. In case of several tracks, the braking force on the second track is equal to 1/14 maximum L.L (of the second track). The braking force has a great effect on the design of the towers and also on the abutments and piers supporting the fixed bearing of bridges (hinged bearing).

In railway bridges a single force 6t (without impact) is taken normal to the track at top rail level and in position giving maximum stresses. The stresses due to the lateral shock of locomotive wheel are considered in the design of:-

Additional stressed will be exists due to erection by cantilever method, so it must be considered during the design of bridge, also the allowable stresses are increased by percentage of 25 %, (0.58Fy 1.25 = 0.73 Fy) (2.5P8).

If the erection of the bridge is done by the cantilever method, the biggest possible forces in the members during the erection must be considered in the design of these members. A higher working stress may be used (or 0.73 y) than for the complete bridge.

It is used in cases of simple beam bridges. Where a loaded ship carries the structure up to the site of erection, then the loads are removed slowly till the structure has in its required erection level.